(A) Heretofore, a curable ink composition, which is cured by actinic rays such as ultraviolet rays and electron rays, or by heat, has been utilized in practice for various applications such as paints, adhesives and printing inks onto such as plastic, paper, wood and inorganic materials, as well as printed circuit board and electric insulation related matters. In recent years, as for printing inks, paints and adhesives among them, further improvement of weather-proofing and adhesion capability has been desired. Further, as ink-jet ink utilizing them, photo-curable ink, which is cured by actinic rays such as ultraviolet rays, is known. An ink-jet method utilizing this ultraviolet curable ink has attracted attention in recent years with respect to relatively low odor, rapid drying and the capability of recording on a recording material without an ink absorptive property, and ultraviolet curable ink-jet ink has been disclosed in such as JP-A 6-200204 (hereinafter, JP-A refers to Unexamined Japanese Patent Application Publication No.) and Japanese Translation of PCT International Patent Application Publication No. 2000-504778.
With respect to these photo-curable inks, an ink employing a radical polymerizing compound, and further an ink employing a cationic polymerizing compound which does not exhibit a polymerization inhibition effect due to oxygen, have been studied (for example, please refer to Patent Documents 1-4).
However, in the case of forming an image by use of ultraviolet curable ink-jet ink, insufficient curing tends to be caused when a large an ink quantity is to be cured during one time ultraviolet irradiation. This is because of deterioration of the interior curing capability due to the increased ink layer thickness. However, it is necessary to increase the ink quantity per one time ejection to improve productivity, however, insufficient curing tends to result due to a heavier ink layer thickness. Further, this problem is more significantly exhibited in the case of a line printer.
(B) An ultraviolet curable ink-jet recording method can perform image formation on various substrates as described before and is increasingly attracting attention. In the ink-jet recording method, there are a serial recording method, which forms an image by dividing the image and overlaying the thinned-out images, and a line recording method, which forms an image with a single pass, and both methods are also realized in an ultraviolet curable ink-jet recording method.
In the case of forming a color image by the ultraviolet curable ink-jet recording method, there are an exposure method to irradiate ultraviolet rays for each color to be consecutively cured and an exposure method to irradiate ultraviolet rays onto plural colors at once, in both the serial recording method and the line recording method. Since in an apparatus to irradiate ultraviolet rays onto each color requires plural light sources, a rise in apparatus size and overall cost up result.
The ultraviolet curable ink-jet recording method is suitable for high speed printing because ink is dried instantly by irradiation of ultraviolet rays. However, the uncured ink quantity after irradiation of ultraviolet rays necessarily increases when a higher printing speed is employed. In a serial print method, it is the case that recording is performed with a reduced number of passes.
In Patent Documents 5 and 6, since bleeding among multi-colors tends to be caused when the ink volume is large, proposed is a method in which the ink adhesion quantity of the finally formed image is limited by utilizing inks of seven colors comprising Y, M, C, K, R, G, and B, which includes secondary colors. This is a technology proposed to prevent color mixture of ink, since the ink absorption rate by a substrate decreases with increased deposited ink quantity. In Patent Documents 5 and 6, there is no description concerning ultraviolet curable ink.
In an ultraviolet curable ink-jet recording method, since ink is cured on the substrate surface without being allowed to be absorbed by the substrate, no problems are caused as hitherto with respect to ink absorption. However, when ultraviolet rays are irradiated onto drops of large ink volume, ultraviolet rays, which contributes to a chemical reaction, do not penetrate to the interior, resulting in deteriorated interior curing and poor adhesion of the ink to the substrate, which is a characteristic problem of ultraviolet curable ink. Further, depending on ink composition and adhered ink volume, only the surface is cured to cause stress between the interior and the surface resulting in generation of curing wrinkles. These phenomena are significant with black ink which has a high light blocking effect against ultraviolet rays. Further, these phenomena also vary greatly depending on the added amount of a polymerization initiator which contributes to curing sensitivity, and absorption of ultraviolet rays is increased when the amount of a polymerization initiator is increased to increase curing sensitivity, which often deteriorates the interior curing capability.
Surface texture will be increased when ink deposition volume is increased due to mixing of colors, and not only surface texture but also visual sensation of quality such as glossiness differs between the highlighted areas and the shadowy areas.
These phenomena tend to be caused with ink having a high light blocking effect against ultraviolet rays depending on such factors as types and density of the colorant utilized in the ink, and ink having a high additive rate of a polymerization initiator, however, these phenomena also vary depending on the type of reaction. Specifically, in cationic polymerizable ink, which exhibits no polymerization inhibition to oxygen and exhibits a high surface curing capability, the above-described phenomena are significant factors which require improvement.
[Patent Document 1] JP-A (hereinafter, JP-A refers to Unexamined Japanese Patent Application Publication No.) 2001-220526 (claims, examples)
[Patent Document 2] JP-A 2002-188025 (claims, examples)
[Patent Document 3] JP-A 2002-317139 (claims, examples)
[Patent Document 4] JP-A 2003-55449 (claims, examples)
[Patent Document 5] JP-A 10-44473
[Patent Document 6] JP-A 8-244254